I. Field of the Invention
The present invention relates to photodetectors and methods for making such photodetectors.
II. Description of Related Art
In avalanche photodiodes (APDs) or photodetectors, incoming light is used to generate carriers (i.e., free electrons or holes) that are collected as current. Semiconductor materials are selected for photodiodes based upon the wavelength range of the radiation that is desired to be utilized or detected. APDs are operated at high reverse-bias voltages where avalanche multiplication takes place. The multiplication of carriers in the high electric field depletion region in these structures gives rise to internal current gain. In linear mode operation, the output photo-induced current within the APD is linearly proportional to the illuminating photon flux, with the level of gain increasing with reverse bias. Importantly, the dark current that flows through the APD also tends to increase with increasing reverse bias. When biased sufficiently above the breakdown voltage, commonly referred to as excess bias, the APD can have sufficient internal gain so that an incident photon can induce a large and self-sustaining avalanche. This operating scheme is often referred to as Geiger mode and the diode along with enabling circuitry may be referred to as a single photon-counting avalanche photodiode (SPAD). In this mode, a non-photogenerated carrier may also be excited leading to avalanche current that is referred to as a dark count. In practice, the detection efficiency and dark count rate for the SPAD increase with increasing excess bias.
High sensitivity deep ultraviolet (DUV) photodetectors operating at wavelengths shorter than 280 nm are useful for various applications, including chemical and biological identification, optical wireless communications, and UV sensing systems. Often, these applications require very low light level or single photon detection and, as a result, photomultiplier tubes (PMTs) are widely used. However, in addition to being large and fragile, PMTs require the use of expensive filters to limit the bandwidth of detection. While APDs can be more compact, lower cost and more rugged than the commonly used PMTs, commercially available devices such as silicon (Si) single photon counting APDs (SPADs) have poor DUV single photon detection efficiency.
Group III-nitride avalanche detectors can presumably be widely functional between 200 nm and 1900 nm (i.e. infrared to ultraviolet radiation). Aluminum gallium nitride (AlxGa1-xN) photodetectors can take advantage of a sharp and tunable direct band gap to achieve high external quantum efficiency and avalanche multiplication in AlxGa1-xN based p-i-n diodes has been reported. See, e.g., L. Sun, J. Chen. J. Li, H. Jiang, “AlGaN Solar-blind Avalanche Photodiodes With High Multiplication Gain,” Appl. Phys. Lett., 97, (191103) (2010) and P. Suvarna, M. Tungare, J. M. Leathersich, P. Agnihotri, F. Shahedipour-Sandvik, L. D. Bell, and S. Nikzad, “Design and Growth of Visible-Blind and Solar-Blind III-N APDs on Sapphire Substrates,” J. Electron. Mater. 42, 854 (2013)), both of which are herein incorporated by reference.
However, this approach is limited by the difficulty in doping high AlN mole fraction alloys p-type, and a very large breakdown electric field for high AlN mole fraction that implies higher voltage operation and greater susceptibility to dark current associated with defects in the material. Therefore, a need remains for low cost, compact, high sensitivity, low dark current/dark count rate photodetectors operating in the ultraviolet spectrum.
Silicon Carbide (SiC) photodetectors have emerged as an attractive candidate for DUV pin photodetectors and APDs due to their very low dark currents, small k factor, and high gain. State-of-the-art SiC APDs employing a recessed top window exhibit peak quantum efficiency (QE) of 60% at 268 nm and k factor of ˜0.1, and dark current of 90 pA at a gain of 1000. See, X. Bai, X. Guo, D. C. Mcintosh, H. Liu, and J. C. Campbell, “High Detection Sensitivity of Ultraviolet 4H-SiC Avalanche Photodiodes”, IEEE J. Quantum Electron 43, 1159 (2007), herein incorporated by reference. These dark currents are more than three orders of magnitude lower than what has been reported for AlxGa1-xN based APDs. See, e.g., L. Sun, J. Chen, J. Li, H. Jiang, “AlGaN Solar-blind Avalanche Photodiodes With High Multiplication Gain,” Appl. Phys. Lett., 97, (191103) (2010), herein incorporated by reference. However, the responsivity of these devices diminishes at wavelengths shorter than 260 nm due to increasing absorption and carrier generation in the illuminated doped layer of this device and the short effective diffusion length of minority carriers in this region in the presence of a high density of surface states through which these photogenerated carriers recombine.
In general, the short wavelength response in pin detectors associated with detection of photons having energies much greater than the band gap of a semiconductor having a high density of surface states is hindered by the absorption of these photons near the surface of the heavily doped illuminated layer (p- or n-type). As a result, photo-generated carriers are trapped by surface band bending and are lost to surface recombination; the carrier transport in this layer may be characterized as diffusion, associated with the spatial gradient in photogenerated carriers, with a significantly reduced diffusion length for minority carriers over what would be expected in the bulk that can be described as a shorter effective diffusion length.
A number of groups have explored Schottky and metal-semiconductor-metal (MSM) 4H-SiC photodetectors to address this issue by enabling more efficient collection of carriers through photogeneration of these carriers primarily within the depletion region of the device. A. Sciuto, et al., “High responsivity 4H-SiC Schottky UV photodiodes based on the pinch-off surface effect,” Appl. Phys. Lett. 89, 081111 (2006) (herein incorporated by reference), report a peak QE of 29% at 255 nm for vertical Schottky diodes fabricated on n-type 4H-SiC using the pinch-off surface effect to increase the direct optical absorption area in the detector. X. Xin, F. Yan, T. W. Koeth, C. Joseph, J. Hu, J. Wu, and J. H. Zhao: “Demonstration of 4H-SiC UV single photon counting avalanche photodiode,” Electron. Lett., 41 1192 (2005) (herein incorporated by reference) reported large-area, 2×2 mm, n-4H-SiC Schottky diodes with QE of ˜20% at 200 nm. However, high efficiency single photon counting capable-APDs at wavelengths shorter than 260 nm have not been demonstrated to date. One challenge has been to realize a device design that mitigates the effects of surface recombination in these devices while maintaining sufficiently low dark currents at high bias to allow avalanche breakdown.
Therefore, a need remains for low cost, compact, high sensitivity, low dark current/dark count rate photodetectors that operate in the ultraviolet spectrum.